1. Field of the Invention
The present invention relates to a dielectric ceramic, a method of procuring the same, and a monolithic ceramic capacitor containing the dielectric ceramic and, particularly, to the improved reduction of the thickness of a dielectric ceramic layer contained in the monolithic ceramic capacitor which can be advantageously carried out.
2. Description of the Related Art
In general, monolithic ceramic capacitors are produced as follows.
First, a ceramic green sheet is prepared. The green sheet contains a dielectric ceramic raw material and has an electroconductive material for an internal electrode applied to the surface of the green sheet in a desired pattern. For example, a dielectric ceramic containing BaTiO3 as a major component is used.
Subsequently, plural ceramic green sheets each having the electroconductive material applied thereon are laminated and hot-press-bonded. Thus, an integrated green laminate is prepared.
Next, the integrated green laminate is fired. Thus, a sintered laminate is produced. The laminate is provided with the internal electrodes formed inside the laminate, which are made from the above-described electroconductive material.
Then, an external electrode is formed on the outer surface of the laminate so as to be electrically connected to a specified internal electrode. In particular, the external electrode is formed, e.g., by applying electroconductive paste containing electroconductive metal powder and glass frit to the outer surface of the laminate, and firing.
Thus, a monolithic ceramic capacitor is produced.
Referring to the electroconductive material to form the internal electrode, base metals such as nickel, copper, or the like, which are relatively inexpensive, have been used in many cases in recent years. However, to produce a monolithic ceramic capacitor containing an internal electrode made of a base metal, firing in a neutral or reducing atmosphere is required to prevent the base metal from being oxidized during firing. Therefore, the dielectric ceramic contained in the monolithic ceramic capacitor must have a reduction-proof property.
As a dielectric ceramic which has the above-described resistance to reduction and can form a monolithic ceramic capacitor of which the capacitance—temperature characteristic meets the requirement of Characteristic B of JIS standard, a material containing BaTiO3 as a major component, and oxides of rare earth elements, oxides of Mn, Fe, Ni, Cu or the like, a sintering-assisting agent, and so forth are used as additives.
Referring to the above-described dielectric ceramic, for example, Japanese Unexamined Patent Application Publication No. 5-9066 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 9-270366 (Patent Document 2), Japanese Unexamined Patent Application Publication No. 11-302071 (Patent Document 3), and Japanese Unexamined Patent Application Publication No. 2000-58377 (Patent Document 4) propose the compositions of dielectric ceramics which have a high dielectric constant, a lower temperature-dependent change of a dielectric constant, and a long high-temperature load service life, respectively.
Referring to the structure of the dielectric ceramic, Japanese Unexamined Patent Application Publication No. 6-5460 (Patent Document 5), Japanese Unexamined Patent Application Publication No. 2001-220224 (Patent Document 6), and Japanese Unexamined Patent Application Publication No. 2001-230149 (Patent Document 7) propose dielectric ceramics having a so-called core-shell structure.
Moreover, Japanese Unexamined Patent Application Publication No. 2001-313225 (Patent Document 8) proposes a dielectric ceramic having a so-called core-shell structure in which the core is partially exposed from the shell.
Recently, electronics have been remarkably developed, and simultaneously, the sizes of electronic parts have been rapidly decreased. Moreover, monolithic ceramic capacitors have experienced a trend toward reduction of the size and increase of the capacitance. As regards effective means for realizing small-sizes and large capacitances in monolithic ceramic capacitors, the thickness of a dielectric ceramic layer is reduced, for example. In general, the thicknesses of dielectric ceramic layers contained in such commercially available products are up to about 2 μm. The thicknesses of dielectric ceramics investigated in laboratories are up to about 1 μm. Enhancement of the dielectric constants of dielectric ceramics is important for realizing small-sizes and large capacitances of the dielectric ceramics.
Moreover, an electrical circuit must be operated with high stability, irrespective of variations in temperature. For this purpose, capacitors used in the electrical circuit must be stable against the variation of temperature.
As seen in the above-description, the advent of monolithic ceramic capacitors, of which the temperature-dependent change of the capacity is small, the electrical insulating property is high and the reliability is superior, even if the thickness of a dielectric ceramic layer is reduced, is earnestly desired.
The dielectric ceramic described in Patent Document 1 meets the characteristic X7R specified in EIA Standard, and moreover, exhibits a high electrical insulating property. However, when the thickness of a dielectric ceramic layer is reduced, and specifically in the case in which the thickness is less than 5 μm, especially less than 3 μm, the capacitance-temperature characteristic and the reliability of the dielectric ceramic do not sufficiently meet the demands in the market.
Similarly, the dielectric ceramics described in Patent Documents 2, 3, and 4, are such that the smaller the thickness of a dielectric ceramic layer is to be, e.g., less than 2 μm, the more the capacitance-temperature characteristics and the reliabilities are deteriorated.
Moreover, each of the so-called core-shell type dielectric ceramics described in Patent Documents 5, 6, and 7 comprises a core portion having a ferroelectric property and a shell portion having a paraelectric property. This dielectric ceramic has a superior capacitance—temperature characteristic. However, the shell portion has a low dielectric constant. Thus, the dielectric constant of the whole dielectric ceramic is reduced, due to the existence of the shell portion. The reason is that when plural dielectrics exist in a ceramic, the dielectric constant of the overall dielectric ceramic is substantially equal to a value calculated by addition of the logarithms of the dielectric constants according to the so-called logarithmic mixing rule. Moreover, problems occur in that with the thickness of a dielectric ceramic layer being reduced, the capacitance—temperature characteristic is deteriorated, and also, the reliability is reduced.
For to the dielectric ceramic having a structure described in Patent Document 8, control of construction is carried out using the firing temperature. Therefore, the electrical characteristics of the dielectric ceramic tend to be dispersed. Thus, problems occur in that for a dielectric ceramic layer of which the thickness is reduced, the capacitance—temperature characteristic and the reliability can not be ensured.
As seen in the above-description, if the thickness of a dielectric ceramic layer is reduced so that the size of a monolithic ceramic capacitor can be reduced, the capacitance thereof is increased, and also, the level of an AC signal is maintained at the same value as that before the reduction of the thickness, the electric field strength applied per dielectric ceramic layer is increased, and thus, the capacitance—temperature characteristic is remarkably deteriorated. Moreover, if the thickness of a dielectric ceramic layer is decreased and the DC rated voltage is set at the same value as that before the reduction of the thickness, the electric field strength applied per dielectric ceramic layer is increased, and thus, the reliability is remarkably deteriorated.
Accordingly, the advent of a dielectric ceramic which has a high dielectric constant is desired, which can be used to form a dielectric ceramic layer of which the temperature-dependent dielectric constant is not deteriorated, even if the thickness of the layer is reduced, and which can provide a monolithic ceramic capacitor with a high reliability.